This application relates to water heaters. More particularly, it relates to an instantaneous water heater which uses a high temperature hot water heat source in lieu of steam.
In many hot water heating systems such as institutional systems, cold water is instantaneously heated by steam in a heat exchanger. For practical reasons, the output flow from the heat exchanger is overheated and is much too hot to be used at a hot water tap. Accordingly, the overheated water is blended with cold water in a blending chamber until a mixture having a temperature suitable for the hot water tap is obtained.
There have been several approaches to heat exchanger designs for hot water in existing closed loop combination systems. These approaches can be broadly categorized as follows: (1) storage tank water heaters, (2) semi-instantaneous water heaters, and (3) instantaneous water heaters.
In the first approach, i.e., the storage tank water heater, a heat exchanger is immersed in a relatively large tank. This heat exchanger is usually a tube coil; the tube may be either finned or unfinned. A further characteristic of such a system is that the tankside fluid is relatively quiescent as far as the heat transfer regime is concerned. In the storage tank heater, no effort is made to promote fluid velocity over the heat exchange surface on the tank side; therefore free convection is the predominant tankside heat transfer mechanism. The storage tank heater is therefore characterized by a modest rate of heat transfer relative to the volume of water stored, and hot water demand is met largely by stored capacitance. The best way to plumb such a system is to circulate boiler fluid in the tube coil and store domestic hot water in the tank.
One advantage of the storage tank water heater is inherent temperature stability in the hot water supply due to the large thermal capacitance of the stored hot water. Another advantage is that a large flow rate may be tapped, at least until the tank is drained of hot water and the boiler cannot keep up with the demand. The disadvantage is that a large tank must be used, with the associated cost, bulk, and thermal loss. Sometimes, the boiler fluid is circulated through the tank and the domestic water is plumbed through the immersed tube coil. Unfortunately, this arrangement retains the disadvantages of the storage tank while reaping little of the benefit. The thermal capacitance is not put to good use, since at high hot water draw, heat will not be transferred at a rate sufficient to maintain hot water temperature unless the coil area is made very large.
The second type of heat exchanger design, i.e., semi-instantaneous water heaters typically use a compact forced convection heat exchanger and may or may not include a small storage tank of hot water which provides some thermal capacitance. The tank-heat exchanger system is designed so that heat can also be transferred from circulating boiler fluid to quiescent water in the tank when there is no domestic water flow through the heat exchanger. Therefore, the heat exchanger can operate in two modes: in the flow (forced convection) mode, heat is transferred at a high rate, thereby providing the capability for delivering an endless flow of hot water; in the recharge (free convection) mode, heat is transferred at a lower rate to quiescent water in the tank, thereby maintaining a small volume of stored hot water. There are several advantages related to maintaining this stored volume of hot water as the thermal capacitance dampens out the temperature instabilities. It also permits a looser link between the boiler heating rate and the heating rate associated with the rate of hot water draw, thereby making controller design easier. In fact, with the semi-instantaneous water heater, the flow switch can be eliminated, and hot water temperature in the heater tank can be used as the feedback control variable. The thermal capacitance also eases the boiler cycling problem that can arise from demand spikes.
An instantaneous water heater is a heat exchanger without any appreciable volume, in which heat is transferred from the boiler fluid flowing through on one side to the domestic water flowing through on the other side. Typically, high fluid velocity is maintained on both sides of the heat exchanger, augmenting the heat transfer coefficient and making possible a compact design relative to the heat transfer rate capacity of the unit. Typical of these compact heat exchangers are tube-in-tube and shell-and-tube designs. Operationally, the system must have a way to sense hot water draw.
One advantage of the instantaneous water heater is that no hot water is stored, so that there is no corresponding thermal loss. Past systems have relied on steam as the heating medium. However, current trends in the industry have resulted in a reduction in the number of facilities using steam.
Accordingly, there is a need for an instantaneous water heater which uses high temperature hot water instead of steam as a heat source and that addresses the foregoing difficulties and others while providing better and more advantageous overall results.